The solid oxide fuel cell (SOFC) owns multiple advantages including high efficiency, low pollution, and using fuel gas and thus regarded a highly potential green-energy conversion device. In structure, an SOFC is formed by an anode, an electrolyte, a cathode, and a connection board. In particular, the anode material plays an extremely important role. It can influence the performance of an SOFC directly.
The main function of the anode material is to act as the catalyzer for electrochemical reactions and provide fuel gas as the reaction sites for oxidation reactions. Generally, to be chosen as the SOFC anode material, the following characteristics should be included: (1) high electron conductivity and ion conductivity, where the electrical conductivity should not vary significantly as the partial pressure of oxide varies; (2) high catalyzing activity to fuel gas, high capability of suppressing carbon deposition, and certain tolerance to hazardous gas, such as hydrogen sulfide, in fuel; (3) appropriate porosity; (4) excellent chemical compatibility and matched thermal expansion coefficients with adjacent other components of cell; and (5) excellent chemical stability, structure, and phase stability in an reduction condition.
In practice, Ni/YSZ is the anode material for SOFC adopted most extensively. It has the advantages of excellent ion and electron conductivity, high catalyzing efficiency, and sufficient pores. Nonetheless, it is limited by the operating environment. It requires pure hydrogen as the fuel, resulting in high cost. If hydrocarbon gas is adopted as the fuel, Ni-based anode material induces the problems of sulfur poisoning and carbon deposition and leading to performance degradation of the fuel cell. Accordingly, a novel anode material that can catalyze hydrocarbon gas directly without the costs of carbon deposition and sulfur poisoning has become the emphasis of research.
Currently, it is already known that the strontium magnesium molybdenum oxide material having perovskite structure in the form of A2BB′O6 (Sr2MgMoO6, SMMO; Y. H. Huang et al., Science, 312, 254-257 (2006)) has excellent capability in resisting carbon deposition and sulfur poisoning. It also own outstanding catalyzing activity for hydrocarbon fuels as well as appropriate matching with electrolyte in thermal expansion coefficients. Thereby, it is regarded as the primary choice for the SOFC anode material. Nonetheless, the electrical conductivity of SMMO is not high, which is disadvantageous to be applied as the SOFC anode material. Fortunately, the A and B sites in the phase structure of the A2BB′O6-type perovskite own strong replacement capability. By replacing the ionic valence electrons at the A and B sites, defects with different valence state in the material can be produced, leading to the mixed ion and electron conductor (MIEC) property in the replaced SMMO material and increasing electrical conductivity. Accordingly, what elements and proportion should be selected to replace in the SMMO material in order to improve the electrical conductivity of the material and make it more suitable to commercial application as a SOFC anode material are the key challenges in this technical field.